DDD: Domain Events — Facts Your Domain Announces

Immutable facts that happened in your domain, named in past tense, published after a state change commits.

When to use

• Notifying other bounded contexts of state changes without coupling
• Triggering side effects (emails, projections, integrations)
• Building an audit trail

Tradeoffs

• Eventual consistency — consumers lag behind the write
• Event schema becomes a contract between contexts (coupling at the schema level)

Go

type OrderPlaced struct {
    OrderID    string
    CustomerID string
    OccurredAt time.Time
}

type EventPublisher interface {
    Publish(event any) error
}

func (o *Order) Place(pub EventPublisher) error {
    o.Status = "placed"
    // publish AFTER state change — caller commits DB tx first
    return pub.Publish(OrderPlaced{
        OrderID: o.ID, CustomerID: o.CustomerID, OccurredAt: time.Now(),
    })
}

Python

from dataclasses import dataclass
from datetime import datetime
from typing import Protocol

@dataclass(frozen=True)
class OrderPlaced:
    order_id: str
    customer_id: str
    occurred_at: datetime

class EventPublisher(Protocol):
    def publish(self, event: object) -> None: ...

def place_order(order: Order, pub: EventPublisher) -> None:
    order.status = "placed"
    # publish AFTER state change — caller commits DB tx first
    pub.publish(OrderPlaced(order.id, order.customer_id, datetime.utcnow()))

Gotcha: Publish events AFTER the transaction commits — not inside it. Otherwise you publish events for transactions that roll back.